Broadband spiral antenna

ABSTRACT

A frequency independent spiral antenna having a plurality of pairs of arms energized with currents equal to the sine of the respective terminal position angles of the arms for substantially removing bandwidth limitations.

United States Patent [191 Voronoff [451 Aug. 6, 1974 BROADBAND SPIRALANTENNA [75] Inventor: George N. Voronoff, San Francisco,

Calif.

[73] Assignee: Textron Inc., Belmont, Calif.

[22] Filed: July 23, 1973 [2]] Appl. No.: 381,707

[52] US. Cl 343/740, 343/853, 343/895 [51] Int. Cl. H0lq l/36 [58] Fieldof Search 343/740, 853, 895

[56] References Cited UNITED STATES PATENTS 3,624,658 ll/l97l Voronoff..343/395 Primary Examiner-Eli Lieberman Attorney, Agent, or Firm-Gregg,Hendricson & Caplan 57 ABSTRACT A frequency independent spiral antennahaving a plurality of pairs of arms energized with currents equal to thesine of the respective terminal position angles of the arms forsubstantially removing bandwidth limitations.

3 Claims, 6 Drawing Figures I i 1 BROADBANDYSPIRAL ANTENNA BACKGROUNDOFINVENTION Spiral antennas commonly denominated as frequency independentantennas are actually only relatively insensitive to frequencyvariations above some minimum frequency. In actual practice it is foundthat mounting of such antennas relatively close to a ground plane or thelike increases frequency dependency which is highly undesirable for manyapplications. This problem is-particularly noted in aircraftinstallations of antennas directly over the aircraft skin which areoften employed with communications or direction-finding equipment. I

There has been developedan antenna described and claimed in U.S. Pat.No. 3,624,658 which provides a major improvement in the field .offrequency independent antennas capable ofproducing a stable radiationpattern over an appreciable bandwidth such as a bandwidth of 4:1 'orgreater."

The present invention provides a further improvement in spiral antennasto substantially eliminate bandwidth limitations. a

I SUMMARY OF INVENTION The antenna of the present invention is comprisedas a plurality of pairs of spiral arms particularly energized tomaximize frequency independence of the antenna. The curvature of thearms of the antenna hereof may be properly delineated as either anArchimedean spiral or as a logarithmic spiralghowever, in the followingdescription of the present invention the term spiral is taken todenominate an antenna having curved arms of these generaltypes.

The antenna of this invention has a generally flat configuration whichmay be closely spaced from a ground plane or the like with the outerends of the spiral arms being terminated as by resistive coupling to theground plane orthe like. The individual arms of the antenna hereof havethe inner terminations thereof spaced equally about a circle surroundingthe center of the antenna. Considering a center line through twooppositely spaced points on such circle the terminalposition angle ofeach antenna arm is taken as the angle between such center line and theinner terminus of the arm. In accordance with the present invention, thespiral arms of the antenna are energized by excitation currents, I of anN arm spiral equal to the sine of the terminal position angle. With theterminal position angle being identified as di excitation is then I, Sinm Where t 1, 2, 3, N. With this energization the only possible modalregions on the spiral will be 1 and N 1 to thus achieve an approximatebandwidth of (N2):1

It will be seen from the foregoing that the frequency independentbandwidth of the present invention increases with increasing numbers ofspiral arms and it is further noted that energization or excitation ofthe antenna arms may be provided by a simple power divider so as tominimize complexity of antenna energization.

DESCRIPTION OF FIGURES The present invention is illustrated as to asingle preferred embodiment thereof in the accompanying drawingswherein:

, 2 1 FIG. 1 is an illustration of a prior art broadband spiral antenna;FIG. 2 is an illustration of radiation patterns of various harmonics ofa spiral antenna;

FIG. 3 is a schematic illustration of a spiral antenna in accordancewith the present invention; H

FIG. 4 is a schematic representation of the inner ends of spiral arms ofan antenna in accordance with the present invention and identifyingvterminal position angle FIG..5 is a schematic representation of anantenna feed network forthe antenna'of FIG. 3;and FIG. 6 is a schematicrepresentation of the antenna of FIG. 3 inside elevation. 1

. DESCRIPTION OF PREFERRE EMBODIMENT F The antenna of the presentinvention comprises a multiarm spiral antennaj having an. even number.of arms from 4 to 20 or more. In FIG. 1 of the drawings there is isillustratedthe prior artantenna of Pat.

' No. 3,624,658 having 6 spiral'armsTh'e first and sec- 0nd arms areconnected together and energization is applied between such connectionand a connection between the fourth and fifth arms as shown. Theopposite third and sixth arms of the antenna are parasitically excited.This prior art antenna produces only Mode 1 and Mode 5 radiation and inaccordance with'the teaching of the above-noted patent, Mode 5 radiationis suppressed by the utilization of an absorber or resistiveterminations at particular antenna arm radius. This antenna has afrequencyindependent bandwidth of the order of 4: 1.The antenna of the,present invention may. be physically configured inmuch the same manneras the antenna of FIG. 1; however, the present invention is not limitedto a 6 arm-spiral and, furthermore, is energized in a different manner.

There is illustrated in FIG. 2 the radiationpatterns for different modesor harmonics of energy radiated from a spiral antenna as originallypresented in the publication Frequency-Independent Antennas by Victor H.Rumsey, Academic Press, 1966, page 122. It will be seen that Modelradiation comprises a single lobe ex tending in'one direction from thesource or antenna and for many purposes this type of radiation is highlydesirable. r In order to preserve'these desirable features of Mod 1 in atruly frequency independent manner, all other modes must be eliminatedor at least suppressed to such an extent that their effect on theradiation becomes minimal.

There is illustrated in FIG. 3 an eight arm spiral antenna in accordancewith the present invention and it will be seen that the arms A through Hare equiangular, for example, and equally spaced apart with innerterminal points A1, Bl,etc. ,.which may lie on a circle about the centerof the antenna. In FIG. '4 there are-illustrated these inner terminalpoints in connection with an XY coordinate system having a center at thecenter of the antenna. A center line along the X axis of the coordinatesystem is taken as a reference line for measurement of terminal positionangles 4:. With an eight arm spiral antenna, as illustrated in FIG. 3,and the arms being equally spaced apart, :1) 0 for Al, d) 45 for BI,etc. Obviously with a different number of arms the increase in terminalposition angle between successive arms will be different. It isparticularly noted that the present invention is limited to a spiralantenna having an even number of arms and this even number may be anynumber from 4 to 20 or more, with the example of eight illustratedherein being chosen solely for con venience.

The present invention provides an unobvious and highly advantageousenergization of the spiral antenna hereof in accordance with thelimitation I, Sin

wherein I,, is the excitation current of the Nth arm and is the terminalposition angle of the Nth arm.

It will be appreciated that the relative excitation currents ofdifferent arms of the antenna of the present invention are proportionedin accordance with the sines of the terminal position angles of thearms; however, the absolute value of any excitation current is, in fact,the product of the sine of the terminal position angle of the armmultiplied by a fixed arbitrary current value 1. Thus the excitationcurrent of the Nth am of an N arm spiral antenna in accordance with thepresent invention In the illustrated example of the present inventionthere are provided eight arms in the spiral with the first arm A havinga terminal position angle d) 0. Sin O and thus the excitation 1,, 0.With the arms of the antenna equally spaced apart, it will be seen thatsin (1),, 45, sin (b 90, sin (b 135, sin 4),; 180, etc. In other words,the terminal positions of the antenna arms are separated 45 apart abouta circle and thus excitation of the arms will be as follows:

In this illustrated example the figures set forth above are intended tobe relative, i.e., each of the numbers in the foregoing table aremultiplied by I which is some predetermined basic excitation currentvalue determining the overall power to be transmitted from the antenna.It is furthermore to be noted that in the foregoing table the negativevalues represent currents of opposite phase and thus, for example, theexcitation currents 1 and I are 180 out of phase.

With an eight arm antenna, as illustrated, energization or excitation ofthe arms may be very simply accomplished to attain the excitation valuesof the present invention. This is illustrated in FIG. 5 wherein an inputpower terminal 51 is shown to be connected to a 180 hybrid or balun 52which then provides currents on the two output lines 53 and 54 which are180 out of phase. Across the top of FIG. 5 there are illustrated eightterminals representing the arms A to H of the antenna and labeled withthese letters. It will be seen that terminal arm A is not connected tothe power supply inasmuch as the present invention, as described above,provides for zero excitation current applied to antenna arm A. The line53 extends from the hybrid 52 to a power splitter 56 which may be whollyconventional and which has three output-lines leading to antenna armterminals B, C, and D. This power splitter 56 applies current to theantenna arms B, C and D in the proporations I 0.707, I 1.00 and I D0.707. The other lead 54 from the hybrid 52 extends to another powersplitter 57, also of conventional design, having three output linesconnected to antenna arms or terminals F, G and H. Current is split toprovide excitation currents to these arms in the proportions I 0.707, 11.00 and I 0.707. It will, however, be appreciated that the hybrid orbalun 52 provides a phase difference between currents applied to theoutput lines 53 and 54 thereof and thus the currents to the left of thehybrid may be considered as being defined by a minus sign so that infact I 0.707, 1 1.00 and I 0.707. The antenna arm E is to receive noenergization and thus it will be noted that terminal E is not connectedto the power supply.

, Power supply connections to the eight arms of the illustrated antennaare clearly of a very simple nature employing only conventional circuitelements to split the excitation currents as required by the present invention. It has been determined that with the recited excitationcurrents of the arms of the antenna hereof, the only possible modalregions on the spiral antenna will be 1 and N-l. For an eight arm spiralwhere N 8 there then results only the possibility of Mode 1 and Mode 7excitation. It is believed clear from FIG. 2 that Mode 7 excitation isthe very minor one. This mode, however, can be completly eliminated byresistively loading the spiral at a predetermined radius. It hasfurthermore been determined that the approximate achievable bandwidth is(N2):l and thus, in the present example of this invention where N 8, thebandwidth is 6:1. This in itself will be appreciated to be a majorimprovement in the art.

Although the present invention has been described with respect to aneight arm antenna, it has been noted above that the invention isapplicable with any spiral antenna having an even number of arms whereinsuch number lies in the range of 4 to 20 or more. A spiral antenna, inaccordance with the present invention, having sixteen arms and energizedas set forth above, would have a bandwidth of 14:1 and it will beappreciated that this is, in fact, substantially no bandwidth limitationat all. For practical applications it is generally considered that abandwidth of 6:1 or more is so broad as to comprise substantially nobandwidth limitation.

It is preferable for the antenna arms of the present invention to beresistively terminated to a ground plane such as illustrated in FIG. 6wherein resistive terminations 61 and 62 are schematically shown toextend from antenna arms to a ground plane 63. It is not, however,necessary with the present invention to employ absorbers behind theantenna even though the antenna is formed as a low profile unit, i.e.,is closely spaced to a ground plane or cavity wall. The invention isequally applicable to VHF and UHF applications and will thus be seen tobe highly advantageous for air-borne applications.

Although the present invention has been described with respect to asingle preferred embodiment thereof, it will be appreciated thatvariations and modifications may be made within the spirit of thepresent invention. It is thus not intended to limit the presentinvention to the precise details of description or illustration.

What is claimed is:

1. A broadband frequency-independent spiral antenna comprising meansenergizing said antenna arms comprising a circuit element adapted toreceive electrical current and pass current onto two output lines inopposite phase, and a pair of power dividers connected one in each ofsaid output lines and to separate antenna arms.

3. The antenna of claim 1 further defined by means resistively loadingeach of said antenna arms adjacent the outer ends thereof forsuppressing mode 7 radiation therefrom.

1. A broadband frequency-independent spiral antenna comprising an evennumber of spiral arms equally spaced circumferentially about a centerand having a center line extending through the inner terminals of twoopposite arms, and means energizing said arms in accordance with therelationship In Sin phi n where In is the excitation current of the ntharm of the antenna and phi is the angle between the center line and theinner terminal of the nth arm of the antenna.
 2. The antenna of claim 1further defined by said means energizing said antenna arms comprising acircuit element adapted to receive electrical current and pass currentonto two output lines in opposite phase, and a pair of power dividersconnected one in each of said output lines and to separate antenna arms.3. The antenna of claim 1 further defined by means resistively loadingeach of said antenna arms adjacent the outer ends thereof forsuppressing mode 7 radiation therefrom.